I actually read the Tribunal as the Old Gods going through and corrupting things from their imprisonment after the Titans ran off to go create more order from chaos on other planets next on their journey.

What better way to piss off the Titans than to corrupt their own creations to recreate chaos from the order they'd just installed? The Old Gods *love* chaos and destruction, if an Old God could love something. The Titans knew that there was a chance the Old Gods would attempt regaining control of Azeroth after they left, which was why they created the Keepers, with Loken being the trigger. They had no idea, however, that the Old Gods would actually attack from all sides-- corrupting the Earthen *and* the Keepers, namely Loken-- rather than starting from the bottom and working their way up.

When adventurers (being us players) killed Loken in Halls of Lightning, we essentially tripped the alarm that signaled Algalon to come and kick off the reorigination program. According to Algalon, this sort of thing happens all the time-- things don't go according to plan, the planet's creatures run amok, etc etc-- and it requires pushing restart on the whole thing. When adventurers encounter Algalon and successfully defeat him, he's baffled because no one else had protested when he came to push restart. It just happened. And yet here we are, the imperfect bastards of the Titan's master plan, thwarting their own machinations.

Long story short: The Titans seriously underestimated the power of the Old Gods and their abilities, even while confined and, supposedly, subdued.